arm64: use for_each_of_cpu_node iterator

Use the for_each_of_cpu_node iterator to iterate over cpu nodes. This
has the side effect of defaulting to iterating using "cpu" node names in
preference to the deprecated (for FDT) device_type == "cpu".

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>